Process for separating a liquid gas mixture containing methane

ABSTRACT

A liquid gas mixture containing methane is pumped at a pressure below the critical pressure of methane through a heat exchanger in the top part of a rectifying column to reliquefy methane vapor prior to discharge of the mixture into the lower part of the rectifying column, while liquid enriched in higher boiling components of the mixture accumulating at the bottom of the column is heated. The reliquefied methane withdrawn from the column may be pumped at the pressure desired for its utilization.

United States Patent Streich [4 1 Apr. 18, 1972 54] PROCESS FORSEPARATING A LIQUID 1x333 lsialllllllte ..208/365 IXT RE CONTAINING c m..62/28 b lr rf i ANE 2,692,484 10/1954 Etienne ..62/28 2,731,810 l/l956Hachmuth ..62/l8 [72] Inventor: Martin Streich, Nieder-Eschbach, Ger-2,823,523 2/1958 Eakin t t --6 /28 many 2,952,984 9/1960 Marshall......62/27 3,362,175 1 1968 B 2 2 [73] Assignee: Messer Griesheim GmbH,Frankfurt, Ger- I urns 6 f 8 many Primary Examiner-Norman Yudkoff [22]Filed; p 1 19 AssistanzExaminerA. F. Purcell Attorney-Paul W. Garbo [21]Appl. No.: 784,102

[57] ABSTRACT Foreign PP Priority Data A liquid gas mixture containingmethane is pumped at a pres- Dec 15 1967 Germany p 5 5 6093 sure belowthe critical pressure of methane through a heat exchanger in the toppart of a rectifying column to reliquefy 52 U.S. Cl ..62/28, 62/34,62/23, methane vapor Prior to discharge of the mixture into the lower62/26 part of the rectifying column, while liquid enriched in higher 5 1Int. Cl ..F29j 3/00, F29j 3/02 boiling components of the mixtureaccumulating at the [58] Field of Search ..62/23, 24, 27, 28, 31, 40;tom of the column is heated- The reliquefied methane 260/676 203/340, 35 withdrawn from the column may be pumped at the pressure desired forits utilization. [56] References Cited UNITED STATES PATENTS 2 Claims, 1Drawing Figure 1,853,743 4/1932 Pollitzer ..62/24 mrnoesn NATURAL GASMETHANE L FRACTION 9 10 11 12 18 w 13 ETHANE 1 M 25 1 PROPANE BUTANE 1g1 Hu l/1 5R 23 HYDROCARBONS PROCESS FOR SEPARATING A LIQUID GAS MIXTURECONTAINING METHANE BACKGROUND OF THE INVENTION This invention relates toa process for the separation of a liquid gas mixture containing methane,other low-boiling hydrocarbons and nitrogen. In particular, the processis directed to the separation of liquid natural gas.

The process of the invention is applicable where either the specificgravity or the calorific value of the gas mixture is to be adjusted to apredetermined figure or where the separation and recovery ofhydrocarbons heavier than methane is desirable. After separation by thisprocess, the methane-enriched fraction is fed to a pipeline at highpressure. The process of this invention is particularly suitable for thecomplete or partial separation of valuable heavy hydrocarbons such asethane, propane, etc., from liquid natural gas.

German Pat. No. 1,122,560 discloses a process for separating a liquidgas mixture consisting of methane and heavier hydrocarbons into methaneand heavier hydrocarbons in a rectifying column. In that process, themethane-enriched gas mixture accumulating in the column top is partiallyreliquefied by heat exchange with the incoming liquid gas mixture. Thecondensate serves as reflux for the column while the portion of themethane-enriched mixture that remains gaseous is expanded and suppliedto the consumer. Contrary to this prior process, it is the object of thepresent invention to supply the separated methane at high pressure,i.e., a pressure which is higher than or close to the critical pressureof methane.

SUMMARY OF THE INVENTION In accordance with this invention, a liquid gasmixture containing methane and heavier hydrocarbons, preferably liquidnatural gas, is pumped into a rectifying column at a high pressure belowthe critical pressure of methane. Liquid enriched in heavierhydrocarbons accumulating in the sump of the column is heated, and thevapor or gaseous phase consisting mainly of methane rises to the top ofthe column. The basic feature of the process of this invention is thatthe gaseous phase of the mixture consisting mainly of methane and risingto the column top is cooled by heat exchange with the liquid gas mixturesupplied to the column so as to reliquefy substantially completely themethane content of the gaseous phase.

A special advantage of this process is, therefore, the ex tremely lowenergy consumption. The illustrative embodiment of the invention whichfollows will further clarify the novel process.

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE shows a process forseparating natural gas by the method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT Liquid natural gas containing anappreciable quantity of nitrogen is pressurized to rectificationpressure by pump 1 and conducted through line 2 and cooling coils 3 and4 in the top portion of rectifying column 5. Then it flows throughcooling coil 6 of ethane column 7, is heated in heater 8 and enters thelower portion of rectifying column 5. There, the liquid natural gas isseparated into a gaseous phase containing methane and nitrogen whichrises to the top of the column, and a liquid phase containing theheavier hydrocarbons which accumulates in the column sump. Methane andnitrogen are condensed by cooling coils 4 and 3. Part of the liquefiedmethane serves as reflux for rectifying column 5, the other part iswithdrawn from rectifying column 5 through line 9, is pressurized to thedesired pressure by pump 10 and is heated in heaters 11 and 12. Theregasified natural gas is conducted through line 13 to the consumer. Thenitrogen which remains gaseous is removed from column 5 through line 14.

The liquid in the sump of column 5 is. heated by heater 15.

The sump liquid consisting of hydrocarbons heavier than methane 1Swithdrawn from column 5 through line 16, ex-

panded through throttle valve 17, and fed to ethane rectifying column 7.Separated gaseous ethane leaves ethane column 7 through line 18. Theportion of the sump liquid which remains liquid in ethane column 7 ispassed through line 19 and throttle valve 20 and fed to propanerectifying column 21. Propane vapor is separated, condensed by outsiderefrigeration, and removed from propane column 21 through line 22. Theremaining liquid containing butane and heavier hydrocarbons is removedfrom the bottom of propane column 21 through line 23. By way of line 24and control valve 25, heavier hydrocarbons can be withdrawn from column5 and added to the methane product stream in line 9, e.g., to regulatethe calorific value of this product stream.

A distinct advantage of this process is that the free energy containedin the liquid natural gas is utilized for the compression and for theseparation of the natural gas. Therefore, the energy consumption of theprocess is negligibly small. Essentially, energy is needed only for thetwo pumps 1 and 10. Ambient heat can be used for heaters 8, l1, l2 and15.

What is claimed is:

l. A process for separating a high pressure liquid methaneenrichedproduct from liquid natural gas containing ethane, other low-boilinghydrocarbons and nitrogen, which comprises pumping a stream of saidliquid natural gas to rectification pressure and through first heatexchange passages in the upper portion of a first rectifying zonemaintained at high pressure below the critical pressure of methane andthrough second heat exchange passages in the upper portion of a secondrectifying zone maintained at high pressure below the pressure in saidfirst rectifying zone, then heating said stream externally of said firstand second rectifying zones to a temperature just below the boilingpoint thereof, discharging the heated stream into the lower portion ofsaid first rectifying zone, heating by indirect heat exchange the liquidaccumulating in the bottom of said first rectifying zone, withdrawingfrom said bottom said liquid enriched in said ethane and otherlow-boiling hydrocarbons and passing it without further heating througha throttle valve into said second rectifying zone, reliquefyingsubstantially all of the resulting methane vapor in said firstrectifying zone by contact with said first heat exchange passages,withdrawing reliquefied methane as said high pressure liquidmethane-enriched product from said upper portion of said firstrectifying zone, discharging a nitrogen-enriched gaseous product fromthe top of said first rectifying zone, and discharging anethane-enriched gaseous product from the top of said second rectifyingzone.

2. The process of claim 1 wherein the high pressure liquidmethane-enriched product is withdrawn by pumping to a delivery pressurehigher than the high pressure maintained in the first rectifying zone.

2. The process of claim 1 wherein the high pressure liquidmethane-enriched product is withdrawn by pumping to a delivery pressurehigher than the high pressure maintained in the first rectifying zone.